The quality and performance of a water well can deteriorate for a number of reasons. The water quality can be adversely affected, the aquifer characteristics can degrade, operational procedures can be poor, or the well can be improperly designed or constructed. Corrosion, encrustation and biological fouling of wells are recognized problems that can lead to degradation in the well performance.
Biological fouling is a particularly common cause of well deterioration. It has been found that 75% to 80% of wells that experience deterioration have a high level of bacterial activity. In most wells that are subject to bacteriological plugging, bacterial slimes are found together with inorganic precipitates (such as iron and manganese oxides and hydroxides) and fines (such as silt, clay and/or sand). The bacterial slime typically acts as a glue which traps and holds the fines and chemical precipitates together and thus promotes the formation of biological masses that can substantially reduce the water yield of the well and deteriorate water quality.
Biological fouling in groundwater supply systems creates numerous problems that adversely affect the well performance. For example, biofouling results in increased frictional resistance and decreased flow area through the well screens. Voids in the gravel pack around the well and elsewhere in the aquifer can become plugged and decrease the flow capacity of the aquifer. The well intake screen can be plugged, and the screen, casing and pump column pipe are subjected to increased corrosion. Restricted flow through the pump can cause operational problems, and the pump impeller and discharge piping can become clogged by biological masses. Finally, biofouling degrades the water quality in its taste, color and odor.
Although there are many different physical and chemical factors that influence the rate of aquifer plugging, the availability of oxygen is the most important factor in stimulating the biological growth activity that is the principal cause of biomass formation. Most of the bacteria that contribute to well plugging are aerobic and require oxygen for growth and development. Also, organic compounds are necessary to support microbial growth. Because organic material is often concentrated at air-water or solid-water interfaces, reduction of these interfaces can reduce the degree of biological fouling of a well.
Prior to drilling of a well, the water in the aquifer may be deficient in nutrients to support significant growth of bacteria of the type that creates biofouling. However, construction of the well provides increased air and the pumping of water from the well increases the aquifer water flow enough to bring nutrients to the well area to establish a growth environment for bacteria.
Air can enter the aquifer by diffusing down the well column, by the backflow of water when the pump is stopped, by diffusion both inside and outside of the well casing if the casing is improperly grout-sealed, and by infiltration to the water table during draw down. The enhanced water turbulence caused by pump operation increases the level of dissolved oxygen in the water. Consequently, the dissolved oxygen is increased at increased pumping rates, and the growth of aerobic bacteria is promoted accordingly.